Transmission modes

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Ebola Virus Disease, also commonly refereed to as Ebola or EVD for short, is spread through direct contact (through broken skin or mucous membranes in, for example, the eyes, nose, or mouth) with blood or body fluids (including but not limited to urine, saliva, sweat, feces, vomit, breast milk, and semen) of a person or animal who is sick with Ebola, objects (like needles and syringes) that have been contaminated with the virus. [1] [2]


Aerosol and Droplet[edit]

The 2007 updated CDC Healthcare Infection Control Practices Advisory Committee (HICPAC) infection control guidance defines droplet transmission as a form of contact transmission in which respiratory droplets carrying infectious pathogens transmit infection when they travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces (nasal mucosa, conjunctivae, and less frequently, the mouth) of a recipient, "generally over short distances, necessitating facial protection." [3]

For diseases classified as being transmitted by the droplet route, surgical masks are recommended to protect the mouth and nose. The guidance also states that although 5 µm (Micrometers) has traditionally been defined as the particle size break point distinguishing between larger particles (droplet transmission) and smaller particles (airborne transmission), observations of particle dynamics have demonstrated that a range of droplet sizes, including those with diameters of 30 μm or greater, can remain suspended in the air. Influenza virus is an example of a pathogen transmitted by the droplet route.

The HICPAC guidance defines airborne transmission as dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance. It states that microorganisms carried in this manner may be dispersed over long distances by air currents and may be inhaled by susceptible individuals who have not had face-to-face contact with (or been in the same room with) the infectious individual.

Furthermore, preventing the spread of pathogens by the airborne route requires the use of special air handling and ventilation systems, such as airborne infection isolation rooms (AIIRs) to contain and then safely remove the infectious agents. In addition to AIIRs, respiratory protection with a NIOSH-certified N95 or higher level respirator is recommended for HCP entering the AIIR to prevent acquisition of airborne infectious agents. Mycobacterium tuberculosis is cited as an example of a pathogen transmitted by the airborne route.

In spite of the distinction made between droplet and airborne transmission, current knowledge of aerosols indicates that there is no clear line differentiating droplet and airborne transmission, as currently defined, on the basis of particle size. Coughing, sneezing, talking, exhalation, and certain medical procedures generate respiratory particles in a wide range of sizes -- not just very large droplets that launch directly to the mucosal surfaces or drop to the floor. In addition, particles begin to evaporate and become smaller immediately upon emission, and particles ranging from very small up to 100 μm can be inhaled by persons in the near vicinity of the source. [4]

In the current infection control paradigm, airborne transmission is synonymous with long-range transmission of pathogens that can be inhaled and require special air handling to contain. This contrasts with droplet transmission, in which infectious particles are thought to be deposited on a mucous membrane, are not inhaled, and do not require special air handling. However, the association of droplet exposure with infection is confounded by inhalation exposure because close contact with infectious people permits droplet exposure but also maximizes inhalation exposure. Therefore, it is incorrect to conclude that because long-range transmission of infection is not observed, a pathogen is transmitted only by the droplet route.

Absence of long-range transmission, as demonstrated for tuberculosis and measles, does not mean that a pathogen cannot be transmitted by inhalation. As currently defined, the terms "droplet" and "airborne transmission" are inadequate to describe aerosol transmission by inhalation at short range. [5]

Blood[edit]

At the peak of illness, an Ebola patient can have 10 billion viral particles in one-fifth of a teaspoon of blood. That compares with 50,000 to 100,000 particles in an untreated H.I.V. patient, and five million to 20 million in someone with untreated hepatitis C. [6]

Dogs[edit]

During the 2001-2002 outbreak in Gabon, [the NHI] observed that several dogs were highly exposed to Ebola virus by eating infected dead animals. To examine whether these animals became infected with Ebola virus, [the NHI] sampled 439 dogs and screened them by Ebola virus-specific immunoglobulin (Ig) G assay, antigen detection, and viral polymerase chain reaction amplification. Seven (8.9%) of 79 samples from the 2 main towns, 15 (15.2%) of 99 samples from Mekambo, and 40 (25.2%) of 159 samples from villages in the Ebola virus-epidemic area had detectable Ebola virus-IgG, compared to only 2 (2%) of 102 samples from France. Among dogs from villages with both infected animal carcasses and human cases, seroprevalence was 31.8%. A significant positive direct association existed between seroprevalence and the distances to the Ebola virus-epidemic area. This study suggests that dogs can be infected by Ebola virus and that the putative infection is asymptomatic. [7] However, the CDC reports no current reports of dogs or cats becoming sick with Ebola. [8] The World Small Animal Veterinary Assn. recommends that pets exposed to Ebola be quarantined and tested. [9]

Wildlife-to-Human[edit]

Ebola can infect mammals and likely first spread to humans from bats in rural Africa. [10]

It is thought that fruit bats of the Pteropodidae family are natural Ebola virus hosts. Ebola is introduced into the human population through close contact with the blood, secretions, organs or other bodily fluids of infected animals such as chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines found ill or dead or in the rainforest. [11]

Bush meat is also a source of transmission from animal to human. Consuming the raw meat of infected bats, apes and/or monkeys can lead to wildlife-to-human Transmission. Animals should be handled with gloves and other appropriate protective clothing. Animal products (blood and meat) should be thoroughly cooked before consumption [12]

During the 2001-2002 outbreak in Gabon, [it was ]observed that several dogs were highly exposed to Ebola virus by eating infected dead animals. To examine whether these animals became infected with Ebola virus, [a team] sampled 439 dogs and screened them by Ebola virus-specific immunoglobulin (Ig) G assay, antigen detection, and viral polymerase chain reaction amplification. Seven (8.9%) of 79 samples from the 2 main towns, 15 (15.2%) of 99 samples from Mekambo, and 40 (25.2%) of 159 samples from villages in the Ebola virus-epidemic area had detectable Ebola virus-IgG, compared to only 2 (2%) of 102 samples from France. Among dogs from villages with both infected animal carcasses and human cases, seroprevalence was 31.8%. A significant positive direct association existed between seroprevalence and the distances to the Ebola virus-epidemic area. This study suggests that dogs can be infected by Ebola virus and that the putative infection is asymptomatic. [13]

After recovery[edit]

Ebola can still be transmitted from a victim's semen, urine, and breast milk for 70 to 90 days. [14], [15]. A 1977 study of the Democratic Republic of Congo's outbreak in Kikwit by Oxford's The Journal of Infectious Diseases found EVD in semen of a Ebola survivor after 61 days. [16], while another study from Oxford during the 1995 Kikwit outbreak in the Democratic Republic of the Congo, found EVD in the semen of a survivor after 82 days. [17]

See also[edit]

Personal Protective Equipment

Notes[edit]

  1. http://www.cdc.gov/vhf/ebola/transmission/
  2. http://www.abc.net.au/news/2014-10-10/how-you-can-and-cannot-get-ebola/5803250
  3. http://www.cdc.gov/ncidod/dhqp/pdf/guidelines/Isolation2007.pdf
  4. http://img.medscape.com/article/741/245/741245-figure.jpg
  5. http://www.medscape.com/viewarticle/741245_3
  6. http://www.nytimes.com/2014/10/14/us/questions-rise-on-preparations-at-hospitals-to-deal-with-ebola.html?_r=1
  7. http://www.ncbi.nlm.nih.gov/pubmed/15757552
  8. http://www.cdc.gov/vhf/ebola/transmission/qas-pets.html
  9. http://www.wsava.org/article/ebola-and-dogs
  10. http://time.com/3480961/ebola-animals-transmission/
  11. http://www.who.int/mediacentre/factsheets/fs103/en/
  12. http://www.who.int/mediacentre/factsheets/fs103/en/
  13. http://www.ncbi.nlm.nih.gov/pubmed/15757552
  14. http://www.motherjones.com/politics/2014/10/how-long-ebola-sperm
  15. http://www.who.int/mediacentre/news/ebola/06-october-2014/en/
  16. http://jid.oxfordjournals.org/content/179/Supplement_1/S28.full
  17. http://jid.oxfordjournals.org/content/179/Supplement_1/S170.long